1. Technical Field
This disclosure relates to a toner used for development of an electrostatic image in electronic photography, electrostatic recording or electrostatic printing, and to a developer, toner container, process cartridge, image forming apparatus and image forming method using that toner.
2. Description of the Related Art
Contact heating-type fixing methods such as hot roller fixing have been widely used as methods for fixing toner. The fixing device used in hot roller fixing is equipped with a heating roller and a pressure roller. In the fixing device a recording medium (recording sheet) that bears thereon a visible image (toner image) is allowed to pass through the pressure contact area (nip area) between the heating roller and pressure roller, melting the toner image for fixation to the recording medium.
With a contact heating fixing method as typified by the hot roller fixing method, since fixing is accomplished by bringing the toner image on a recording sheet into contact with the surface of a heating member (e.g., heating roller) of the fixing unit (contact heating fixing device), it is necessary to prevent an offset phenomenon in which some toner particles of the toner image adhere to the heating member and smear the next recording sheet.
To solve this problem there is a known technique which coats or impregnates the heating roller and pressure roller of the fixing device with a fixing oil such as silicone oil or the like, but in view of demand for smaller fixing devices and lower costs, oilless fixing devices with no fixing oil application mechanism and fixing devices using smaller amounts of fixing oil have been employed. When utilizing this kind of fixing device, a releasing agent is added to the toner as an offset-preventing agent.
In the case of the heating fixing method, it is preferable for the heating temperature to be as low as possible in order to conserve energy, but if the heat characteristics of binder resin that constitute toner is designed too low in order to do so, the toner's heat resistance decreases and problems such as blocking occur. In order to realize both this low-temperature fixing property and heat resistance, it is advantageous to use a polyester resin in the binder resin. Compared to vinyl copolymer resins, polyester resins have low viscosity and high elasticity, so they are superior in low-temperature fixing properties while also having good heat resistance.
However, when a toner having a sufficient quantity of releasing agent that has been added to prevent offset is produced using a conventional pulverization method, a large amount of the releasing agent is exposed to the toner surface, leading to such problems as filming and blocking.
Meanwhile, so-called polymerization methods are known, such as suspension polymerization that involves polymerization of a polymerizable monomer in an aqueous medium, and emulsification aggregation wherein fine particles are produced by emulsion polymerization followed by aggregation. These polymerization methods can achieve a higher releasing agent content than the pulverization method. With regard to suspension polymerization, a structure-controlled toner is proposed that is produced by adding an additional polymerizable monomer for further polymerization after normal toner granulation (see Japanese Patent (JP-B) No. 3195362). In addition, with regard to emulsion aggregation, a structure-controlled toner is proposed that is produced by adding additional emulsified fine particles for aggregation after normal toner granulation by aggregation (see Japanese Patent Application Laid-Open No. 2002-116574).
However, both the suspension polymerization and emulsion aggregation methods accomplish polymerization in a aqueous medium and hence use a vinyl-based copolymer resin is used; thus it is difficult to employ polyester resins that undergo polymerization at high temperatures of around 200° C.
In addition, as a method for creating toner particles using polyester resin, a so-called solution suspension method is known wherein particles are created in an aqueous medium by dissolving a pre-polymerized resin in an organic solvent. In this method, the molecular weight of the resin during preparation becomes the molecular weight of the resultant toner. While it is common to mix a resin having a low molecular weight with a resin having a high molecular weight in order to adjust the toner's thermal properties, when a resin having a high molecular weight is introduced, it results in a problem such as poor toner granulation efficiency due to too high viscosity of solution containing the high molecular weight resin, thereby making it impossible to use a large amount of high molecular weight resin. For this reason, there is no choice but to increase the molecular weight of resin with a low molecular weight, which is disadvantageous for low-temperature fixing.
In order to overcome this problem there is a method that causes a modified polyester resin bearing reactive groups to undergo extension and crosslinking reactions after toner granulation, instead of introducing a resin with a high molecular weight. With this method it is possible to adjust the toner's thermal properties, but control of toner structure is insufficient, and the colorant, releasing agent and the like tend to be exposed to the toner surface. This method further presents a problem particularly in the case of development using a one-component developer that poor charging ability leads to toner deposition on the background of the copy after continuous use.
Accordingly, the current situation is that no toner and related technologies have yet been provided that realize both low-temperature fixing and heat resistance, is excellent in terms of offset resistance, enables control of the toner structure, and has good charging ability and suitability for cleaner-less apparatus without soiling of the developing device and the like, and that it is desired that they be desired to be provided as early as possible.